Crystal structure of strontium perchlorate anhydrate, Sr(ClO4)2, from laboratory powder X-ray diffraction data

The crystal structure of Sr(ClO4)2 is isotypic with its Ca homologue.

The crystal structure of strontium perchlorate anhydrate, Sr(ClO 4 ) 2 , was determined and refined from laboratory powder X-ray diffraction data. The material was obtained by dehydration of Sr(ClO 4 ) 2 Á3H 2 O at 523 K for two weeks. It crystallizes in the orthorhombic space group Pbca and is isotypic with Ca(AlD 4 ) 2 and Ca(ClO 4 ) 2 . The asymmetric unit contains one Sr, two Cl and eight O sites, all on general positions (Wyckoff position 8c). The crystal structure consists of Sr 2+ cations and isolated ClO 4 À tetrahedra. The Sr 2+ cation is coordinated by eight O atoms from eight ClO 4 À tetrahedra. The validity of the crystal structure model for Sr(ClO 4 ) 2 anhydrate was confirmed by the bond valence method.

Chemical context
The alkaline earth metal ions (Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ ) have received attention as ion carriers for next-generation batteries (Wang et al., 2013), and their perchlorates are used as inorganic salts of conventional nonaqueous electrolytes for electrochemical cells in Mg-and Ca-ion batteries (Whittingham et al., 2018;Tchitchekova et al., 2017;Padigi et al., 2015). It is crucial to obtain anhydrous salts to achieve high electrochemical cell performance since hydrated salts can cause unwanted side reactions as a result of increased water content in the nonaqueous electrolyte. Strontium perchlorate is highly hygroscopic and exists in several hydrated forms. So far, Sr(ClO 4 ) 2 Á3H 2 O, Sr(ClO 4 ) 2 Á4H 2 O and Sr(ClO 4 ) 2 Á9H 2 O have been identified by single-crystal X-ray diffraction (Hennings et al., 2014). However, the crystal structure of the anhydrous phase has not been reported to date because of the difficulty in growing single crystals. Previously, we have determined the structures of anhydrous magnesium, barium and calcium perchlorate from laboratory powder X-ray diffraction data (Lim et al., 2011;Lee et al., 2015Lee et al., , 2018. Using the same techniques for the Sr salt, we were able to determine and refine the crystal structure of strontium perchlorate anhydrate.
There are one Sr, two Cl and eight O sites in the asymmetric unit, all on general positions 8c. The crystal structure ( Fig. 1) is composed of Sr 2+ cations and isolated ClO 4 À tetrahedra. The isolated ClO 4 À tetrahedra are slightly distorted and exhibit a range of 105.4 (7)-113.5 (7) for the O-Cl-O angles. The local environment around the Sr 2+ cation is presented in Fig. 2 Lee et al., 2015) polyhedra, and in good agreement with the sum of the ionic radii of the respective alkaline earth metal and oxygen ions (Shannon, 1976).

Synthesis and crystallization
Anhydrous strontium perchlorate was obtained by dehydration of Sr(ClO 4 ) 2 Á3H 2 O (98%, Alfa Aesar). The hydrated Sr(ClO 4 ) 2 powder was ground thoroughly in an agate mortar and added to a glass bottle. The bottle was placed in an oven at 523 K for two weeks under atmospheric conditions and then transferred to a glove-box under an argon atmosphere. For the powder X-ray diffraction measurements, anhydrous Sr(ClO 4 ) 2 was again ground in an agate mortar and placed in a tightly sealed dome-type X-ray sample holder commercially available from Bruker. The dome was double-sealed with vacuum grease to prevent hydration during measurement.

Refinement details
Details of the crystal data collection and structure refinement are summarized in Table 2. Powder X-ray diffraction (PXRD) data for anhydrous Sr(ClO 4 ) 2 were collected from a Bragg-Brentano diffractometer (PANalytical Empyrean) using  Table 1 Selected bond lengths (Å ).

Figure 1
The local environment of the Sr 2+ cation (yellow sphere) surrounded by ClO 4 À tetrahedra (purple). Symmetry codes refer to Table 1.

Figure 2
The crystal structure of Sr( Cu K 1 radiation, a focusing primary Ge(111) monochromator ( = 1.5406 Å ) and a position-sensitive PIXcel 3D 2Â2 detector. The angular range was 10 2 130 , with a step of 0.0131 and a total measurement time of 8 h at room temperature. The PXRD pattern was indexed using the TREOR90 algorithm (Werner, 1990) run in CRYSFIRE (Shirley, 2002) through the positions of 23 reflections, resulting in an orthorhombic unit cell. Systematic reflection conditions suggested the space group Pbca. The crystal structure was determined by a combination of the powder profile refinement program GSAS (Larson & Von Dreele, 2000) and the singlecrystal structure refinement program CRYSTALS (Betteridge et al., 2003). For a three-dimensional view of the Fourier electron-density maps, MCE was applied (Rohlícek & Husá k, 2007). Initially, a structural model with only one dummy atom at an arbitrary position in the unit cell was used. Structure factors were extracted from the powder data and then direct methods were applied to calculate the initial solution of the crystal structure using SHELXS97 (Sheldrick, 2008) run in CRYSTALS, which yielded the Sr site as a starting atomic position. The initial dummy atom model was then replaced with the partial model, and this data was adopted for a Le Bail fit in GSAS. Improved structure factors were then extracted, which were used for the refinement in CRYSTALS. Such processes were iterated until a complete and satisfactory structural model was obtained. Finally, Rietveld refinement in GSAS was employed to complete the structure model, resulting in reasonable isotropic displacement parameters and agreement indices. For the final Rietveld refinement with GSAS, the Sr-O and Cl-O bond lengths were restrained with a tolerance value of 2% with respect to the distances determined from CRYSTALS, which matched reasonably well with the radii sums of Shannon (1976). The final Rietveld plot is displayed in Fig. 3.    (Sheldrick, 2008) and CRYSTALS (Betteridge et al., 2003); program(s) used to refine structure: GSAS (Larson & Von Dreele, 2000); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: GSAS (Larson & Von Dreele, 2000).

Strontium perchlorate anhydrate
Crystal data